Continuing advancements in solid-state lighting technologies, and specifically light-emitting diodes (LEDs), continue to result in remarkable performance improvements when compared to their incandescent and fluorescent counterparts. Generally, LED-based lighting fixtures are more efficient, last longer, are more environmentally friendly, and require less maintenance than incandescent and fluorescent lighting fixtures. Accordingly, LEDs are poised to replace conventional lighting technologies in applications such as traffic lights, automobiles, general-purpose lighting, and liquid-crystal-display (LCD) backlighting.
LED lighting fixtures are driven by a linear (i.e., direct current) driver signal or a pulse-width modulated (PWM) driver signal. Since most lighting fixtures receive power from an alternating current (AC) power source, power conversion must be performed by driver circuitry in order to produce a desired light output from the LED lighting fixture. While the color of light emitted from an LED primarily depends on the composition of the material used to fabricate the LED, the light output of an LED is directly related to the current flowing through the P-N junction of the LED. Accordingly, driver circuitry capable of providing a constant current is desirable for an LED lighting fixture.
FIG. 1 shows conventional driver circuitry 10 for an LED lighting fixture. For context, a power supply 12, an electromagnetic interference (EMI) filter 14, control circuitry 16, and an LED light source 18 are also shown. The conventional driver circuitry 10 includes rectifier circuitry 20, power factor correction (PFC) circuitry 22, and DC-DC converter circuitry 24. The rectifier circuitry 20 is a bridge rectifier including a first rectifier input node 26A, a second rectifier input node 26B, a rectifier output node 28, a first rectifier diode DR1, a second rectifier diode DR2, a third rectifier diode DR3, and a fourth rectifier diode DR4. The first rectifier diode DR1 includes an anode coupled to the first rectifier input node 26A and a cathode coupled to the rectifier output node 28. The second rectifier diode DR2 includes an anode coupled to the second rectifier input node 26B and a cathode coupled to the rectifier output node 28. The third rectifier diode DR3 includes an anode coupled to ground and a cathode coupled to the first rectifier input node 26A. The fourth rectifier diode DR4 includes an anode coupled to ground and a cathode coupled to the second rectifier input node 26B. The first rectifier input node 26A is coupled to a positive output of the power supply 12, which is filtered via the EMI filter 14. The second rectifier input node 26B is coupled to a negative output of the power supply 12, which is also filtered via the EMI filter 14.
The PFC circuitry 22 is a boost converter including a boost input node 30, a boost output node 32, a boost inductor LB, a boost switch QB, a boost diode DB, and a boost capacitor CB. The boost inductor LB is coupled between the boost input node 30 and an intermediary boost node 34. The boost switch QB is coupled between the intermediary boost node 34 and ground. The boost diode DB is coupled between the intermediary boost node 34 and the boost output node 32. Finally, the boost capacitor CB is coupled between the boost output node 32 and ground. The boost input node 30 is coupled to the rectifier output node 28 of the rectifier circuitry 20.
The DC-DC converter circuitry 24 is a flyback converter including a flyback input node 36, a flyback output node 38, a flyback transformer TFB, a flyback switch QFB, a flyback diode DFB, and a flyback capacitor CFB. The flyback transformer TFB includes a primary winding 40 coupled in series with the flyback switch QFB between the flyback input node 36 and ground. Further, the flyback transformer TFB includes a secondary winding 42 coupled between an anode of the flyback diode DFB and ground, wherein the cathode of the flyback diode DFB is in turn coupled to the flyback output node 38. Finally, the flyback capacitor CFB is coupled between the flyback output node 38 and ground. The flyback input node 36 is coupled to the boost output node 32, while the flyback output node 38 is coupled to the LED light source 18. In some cases, an additional switch (not shown) may be coupled between the LED light source 18 and ground, such that the additional switch operates to pulse-width modulate the current through the LED light source 18 in order to generate a desired light output.
In operation, an EMI-filtered AC input voltage from the power supply 12 is received at the rectifier circuitry 20, where it is rectified to generate a rectified voltage. The rectified voltage is then received by the PFC circuitry 22, which performs power factor correction and boosts the voltage of the signal to generate a direct current (DC) PFC voltage. The DC-DC converter circuitry 24 receives the PFC voltage and regulates a driver output current, which is used to drive the LED light source 18. The control circuitry 16, which may be separated into discrete PFC control circuitry, DC-DC control circuitry, and dimming control circuitry in some cases, operates the boost switch QB and the flyback switch QFB to generate a desired driver output current. While effective at generating a driver output current that is suitable for driving the LED light source 18, the conventional driver circuitry 10 shown in FIG. 1 generally suffers from low efficiency due to the use of a flyback converter topology for the DC-DC converter circuitry 24. That is, the isolated nature of the flyback converter restricts the efficiency of the DC-DC converter circuitry 24, thereby increasing the power consumption and heat production thereof.
Notably, the switching components in the conventional driver circuitry 10, (i.e., the boost switch QB, the boost diode DB, the flyback switch QFB, and the flyback diode DFB) are silicon (Si) parts, which further hampers the performance of the conventional driver circuitry 10. Specifically, because of the use of silicon (Si) switching components in the conventional driver circuitry 10, the switching frequency and power handling capability of these components is significantly limited. Accordingly, the acceptable voltage range of the AC input voltage as well as the output voltage and current of the conventional driver circuitry 10 are likewise limited. Since the AC input voltage may vary significantly (i.e. from 208V to 480V depending on the infrastructure of the country in which the lighting fixture is deployed), the limited input voltage of the conventional driver circuitry 10 may result in the need to design separate driver circuitry for each country or region in which the driver circuitry is to be sold or used, thereby driving up the cost of manufacturing. Further, since the power handling capability of silicon (Si) devices is limited, the switching devices must be made large for high power applications, and further may produce excessive amounts of heat, resulting in lighting fixtures that are bulky or otherwise undesirable.
FIG. 2 shows the conventional driver circuitry 10 wherein the DC-DC converter circuitry 24 is a half-bridge LLC converter. The DC-DC converter circuitry 24 thus includes a half-bridge input node 44, a half-bridge output node 46, a first half-bridge switch QHB1, a second half-bridge switch QHB2, a first half-bridge capacitor CHB1, a half-bridge inductor LHB, a half-bridge transformer THB, a first half-bridge diode DHB1, a second half-bridge diode DHB2, and a second half-bridge capacitor CHB2. The first half-bridge switch QHB1 is coupled between the half-bridge input node 44 and a half-bridge intermediary node 48. The second half-bridge switch QHB2 is coupled between the half-bridge intermediary node 48 and ground. The first half-bridge capacitor CHB1, the half-bridge inductor LHB, and a primary winding 50 of the half-bridge transformer THB are coupled in series between the half-bridge intermediary node 48 and ground. A second center-tapped winding 52 of the half-bridge transformer THB is coupled between an anode of the first half-bridge diode DHB1 and an anode of the second half-bridge diode DHB2, while the center-tap of the second center-tapped winding 52 is coupled to ground. The cathode of the first half-bridge diode DHB1 and the cathode of the second half-bridge diode DHB2 are each coupled to the half-bridge output node 46. Finally, the second half-bridge capacitor CHB2 is coupled between the half-bridge output node 46 and ground. The half-bridge input node 44 is coupled to the boost output node 32, while the half-bridge output node 46 is coupled to the LED light source 18.
The conventional driver circuitry 10 shown in FIG. 2 functions in a substantially similar manner to the conventional driver circuitry 10 shown in FIG. 10, substituting the principles of operation of a flyback converter for that of an LLC half-bridge converter. Using an LLC half-bridge converter for the DC-DC converter circuitry results in an increase in the efficiency of the conventional driver circuitry 10, however, such a performance increase comes at the expense of increased complexity, cost, and area. Further, the switching components (i.e., the boost switch QB, the boost diode DB, the first half-bridge switch QHB1, the second half-bridge switch QHB2, the first half-bridge diode DHB1, and the second half-bridge diode DHB2) are also silicon (Si) components in the conventional driver circuitry 10 shown in FIG. 2, which once again results in the same limits on the performance of the circuitry as discussed above with respect to FIG. 1.
Accordingly, there is a need for compact driver circuitry for a solid-state lighting fixture that is capable of delivering a constant output current while operating efficiently over a wide range of input voltages.